I need to take a couple of steps back to show you how I bent the upper longeron to match the angle of the rear stringer that runs from behind F6, all the way back to F12. You can't really bend the upper longeron until it is mounted in place because you don't know the angle to which to bend it. Here is the tool I made to bend mine.
Click on photos to enlarge
I used a scrap 2x4 and cut a slot at one end of it just wide and deep enough to fit the upper longeron into. My 2x4 just happened to be 27" long, but it gave me enough leverage to bend the forward longeron inward.
It is important to have bulkhead F10 in place and notched for the 3/4"x1/16" angles, making sure they are level with the top of the forward longeron. Then temporarily mount the 3/4"x1/16" stringer and bend the forward longeron inward until the angle matches the stringer. A second pair of hands is nice to hold the stringer while you bend the longeron, but not required.